LEACHING AND DISSOLVING PRACTICAL CONSIDERATIONS IN LEACHING OPERATIONS Introduction.—Having discussed briefly the theory of dissolutk consider some of the ways in which the dissolution of solids in liqi; plished in actual practice. The problem is evidently to bring cont. solution in contact with the solid particles; that is, different solution it may not be any less concentrated with the solute than the solution IT; a from around the particles still the mere friction of this displacement sex reduce the thickness of the film surrounding it. Therefore it would seen, the most efficient way to accomplish this is to move the solid through the sol or the solution through the particles or to move both in opposite direcu. while in contact with each other. The different systems of leaching may be listed as follows: (1) Leaching by percolation which means flowing the solution past the stationary solids and separation of the solution thus obtained from the residues or undissolved material. (2) Leaching by agitation in which dissolution is obtained while the solids are held in suspension in the solvent and a certain amount of relative motion is maintained and separation of the solution' later by decantation or filtration. Each type listed above will be discussed showing the equipment involved, comparative costs, and its applications in industrial problems. Leaching by Percolation.—This is a well-established method of leaching in which the material to be leached is placed in a tank equipped with a false bottom. The solvent is added at either the top or bottom of the tank and allowed to percolate through the material, being removed when it has passed through it. These tanks are usually arranged so that a counter-current system is employed, the new solids being added to the last tank and the weak liquor to the first and pumped successively from one tank to another till it reaches the last almost saturated. The dissolution of the solid material will have been completed in the early washes, the later washes being merely washing or displacing the stronger liquors retained by the solid material. This process is well suited to cases where the material is porous and sandy or where the solid material is difficult to handle by other methods but is not applicable to materials which tend to pack into impervious masses. It has been found in practice that regularity in the size of the particles and not their size is the chief factor governing good percolation in such tanks, the idea being that where the particles are of all sizes the small ones pack in the openings between the larger ones thereby clogging up the channels. On the other hand, extremely fine material can be leached economically by percolation if it is preceded by a classification of the material so that the particles in any one vat are practically all of the same size. This then involves another process, namely of classification, in treating most materials and two processes of dissolution, one to treat the very fine sludge and another to treat the sandy matter. For this reason the use of percolation in leaching has been rather restricted in its application. Metallurgical Practice.—Leaching of sands by percolation was very widely used in the United States up to a few years ago in the cyanide treatment of gold ores, and is still largely used in the Transvaal. The reason for its decline was the successful development of processes for the treatment of slimes at lower costs which resulted in the plants treating all their material as slime instead of separate treatments as before for sands and slimes. The question of removing the leached material from the tanks is handled in a number of different ways.